The present invention relates to the field of treatment of fluids, and, in particular, relates to the regeneration of adsorption systems used to remove organic solvents from gas streams.
Although the present invention is described with respect to its use in the printing industry, the invention can be used in other fields, and is not limited to any one application.
The printing industry uses organic solvents that are combined with printing inks. Solvents such as toluene are added to the ink to make the ink flow more readily. After the ink has been deposited on a sheet of paper in the printing operation, much of the solvent evaporates. Because solvents such as toluene are both carcinogenic and explosive, it is necessary to dispose of them properly, and not to allow them to be discharged into the ambient air.
In a typical installation, the toluene released from printing ink is conveyed, with a gas stream, to an adsorption bed made of activated charcoal. The activated charcoal bed traps the toluene, while allowing other gases to pass through. After a period of continuous use, the available sites in the charcoal bed eventually become filled by the trapped toluene, and the adsorption bed loses its effectiveness. To restore the adsorption bed to its original condition, one must remove the trapped toluene, in a process known as regeneration. Regeneration is typically performed by heating the bed so as to drive off the toluene or similar solvents.
In the prior art, it is known to regenerate a charcoal bed with the use of steam. FIG. 1 shows a typical system made according to the prior art. The system includes two activated charcoal beds. Bed 1 is shown as “active”, and bed 3 is shown as being regenerated. The active bed is the one that receives the solvent-containing exhaust, carried by conduit 11, from the printing process, or other process, and its output comprises a cleaned gas, carried by conduit 13, that is substantially free of the solvent being removed. While one bed is active, the other bed is being regenerated. Periodically, the beds are switched, so that the bed that was previously being regenerated becomes the active bed, and vice versa. The dotted line extension of conduit 11 indicates that when the beds are switched, the exhaust gas from the printing or other process goes into bed 3 (which becomes the active bed) instead of bed 1.
The bed 3 being regenerated receives steam that comes from boiler 5. In the prior art, natural gas is typically used as the fuel for the boiler. The hot steam, carried by conduit 15, is passed through the bed 3 being regenerated, and the heat from the steam drives the toluene (or other material trapped by the adsorption bed) out of the bed. The fluid stream exiting the bed being regenerated is then cooled, in heat exchanger 7, so as to liquefy the stream. This fluid includes toluene, and water which has condensed from the steam. The resulting water-toluene mixture passes to separator 9. Because water and toluene do not mix, and water is more dense, the water collects on the bottom and the toluene sits on top. One can then easily separate the toluene from the water, such as by decanting. The remaining water is substantially free of toluene, and can be stripped of volatile organic compounds (VOC) before being conveyed to a suitable conduit for waste water discharge.
The dotted line extension of conduit 15 shows that when the beds are switched, the steam is directed into bed 1, which becomes the bed being regenerated.
A major disadvantage of the arrangement of FIG. 1 is that the system requires a large amount of natural gas, or coal, or other fuel, to heat the water to make the steam. As the price of natural gas rises, it becomes prohibitively expensive to practice the above-described solvent treatment process.
The present invention solves the above-described problem by providing a method and system which uses heated nitrogen for regeneration of an adsorption medium, thereby avoiding the use of steam. The present invention includes means for optimizing the use of energy, such that the process is carried out with substantially improved efficiency, as compared with systems of the prior art.